Understanding Basic RF Terminology

This page covers basic RF terminology useful for RF design, development, and testing.

It covers RF terms which include power, relation between dBm and dBW, VSWR, Gain, Return Loss, relation between VSWR and Return loss, Insertion Loss, Reflection coefficient, harmonics, spurious, phase noise, group delay, frequency stability both short term and long term, 1dB compression point(P1dB), 3rd order intercept point(TOI), LNB(Low Noise Block converter), BUC(Block Up Converter), Sensitivity, AM-AM conversion, AM-PM conversion, EMI, EMC, Susceptibility and more.

Power

It is usually the measurement of IF signal and RF signal, which is expressed in dBm or dBW. Power levels at IF output of a modem will usually be lower and typically fall in the range from -20dBm to 0dBm. Power levels at SSPA or HPA are usually higher and typically fall around 20 to 40W and sometimes 50-100W and hence measured in dBW.

dBm is the ratio (in dB) of power relative to 1 mW:

$dBm = 10 * Log(P)$, where P is power in mW.

dBW is the ratio (in dB) of power relative to 1W:

$dBW = 10 * Log(P)$, where P is power in watts.

$0dBW = +30dBm$

VSWR

Stands for Voltage Standing Wave Ratio and is a measure of the accuracy of impedance matching at a point of connection. It is expressed as a ratio e.g. 1.3:1. VSWR is usually used at microwave frequencies. Return Loss is most commonly used at IF frequencies.

$RL = 20 * Log(\frac{VSWR+1}{VSWR-1})$

Gain

It is expressed in dB and is the ratio of output power of an amplifier device to the input power.

Return Loss

Function is similar to VSWR but expressed in dB.

Insertion Loss

It is expressed in dB and is the ratio of output power of an attenuating device to the input power.

Reflection Coefficient

It is the ratio of reflected wave to incident wave at point of reflection on transmission line. This value varies from -1 (for short load) to +1 (for open load), and becomes 0 for matched impedance load.

Reflection coefficient (r) = $\frac{Z_n - 1}{Z_n + 1}$,

Where: $Z_n = \frac{Z}{Z_0}$ $Z_n$ = normalized load impedance $Z_0$ = characteristic impedance of line (Ohm) $Z$ = load impedance (Ohm)

Harmonics

All frequency converters and signal sources produce frequencies at integer multiples of input frequencies, which are called harmonics.

Spurious

All frequency converters and signal sources produce frequencies at non-integer multiples of input frequencies, which are called spurious.

Phase Noise

It is measured in the frequency domain and is expressed as a ratio of signal power to noise power measured in a 1Hz Bandwidth at a given offset from the desired signal.

Group Delay

Delay for the signal to transit from input to output for Device under Test (DUT). This is caused due to reactive elements used within the device. It is proportional to the change in slope of the phase shift response versus frequency curve. For example +/-2ns from 824 to 835 MHz.

Frequency Stability

It is the change in frequency of the Local oscillators, frequency converters and synthesizers mainly due to aging and temperature. They are two types short term and long term and measured in PPM/day or PPM/year respectively.

PPM: Parts per million, usually used to express frequency stability of Local oscillators.

1dB Compression Point (P1dB)

The 1dB Gain Compression point is the output power at which the input/output transfer response of an SSPA departs from a straight line by 1dB. Higher it is good for the system.

3rd Order Intercept Point (TOI)

It is a measure of linearity which describes the amount of third order harmonic that can be expressed in a device. It is referred to the input and output of a module.

TOI in dBm = (Input signal levels in dBm) + (distortion products (dBc)/2)

P1dB versus TOI

LNB

It is used as abbreviation for Low Noise Block converter, which is combination of LNA and Down converter.

BUC

It is used as abbreviation for Block Up Converter, which is combination of Up converter and Power Amplifier.

Sensitivity

Measured in receiver. Sensitivity in a receiver is normally taken as the minimum input signal (Smin) required to produce a specified output signal having a specified signal-to-noise (S/N) ratio and is defined as the minimum signal-to-noise ratio times the mean noise power

$S_{min} = (S/N)_{min} * k * T_o * B * (NF)$

AM-AM Conversion

AM-to-AM conversion measures the amount of undesired amplitude deviation (AM) that is caused by amplitude variations (AM) of the system. It is a critical parameter in systems where amplitude modulation is used, such as QAM. Un-intentional amplitude variations (AM) are caused due to Power supply ripple, Thermal drift and Multipath fading.

AM-PM Conversion

AM-to-PM conversion measures the amount of undesired phase deviation (PM) that is caused by amplitude variations (AM) of the system. AM-to-PM conversion is a critical parameter in systems where phase (angular) modulation is used, such as FM, QPSK and 16QAM. AM-to-PM conversion is usually defined as the change in output phase for a 1-dB increment in the power-sweep applied to the amplifier’s input.

EMI

Extend to which one device radiated field influences the proper functioning of another device. It is caused by undesirable radiated electromagnetic fields or conducted voltages or currents.

EMC

Extend to which electrical appliances can withstand each others radiated fields. The goal of EMC is to minimize the influence of electric noise.

Susceptibility

The inability of equipment/systems to perform without degradation in the presence of an electro-magnetic disturbance. The threshold of susceptibility is the level of interference at which the DUT begins to show degradation in performance. This is often frequency dependent.